Kinetic hydrogen isotope effects in the halogenation of 2-oxocyclohexanecarboxylic acid and its methyl ester

Abstract

Measurements have been made on the rates of bromination of 2-oxocyclohexanecarboxylic acid and of bromination and iodination of methyl 2-oxocyclohexanecarboxylate. The observed rates are independent of the nature and concentration of the halogen involved and represent the rates of ionization or enolisation of the substrates. The reaction are base-catalysed and catalytic constants and kinetic hydrogen isotope effects for some carboxylate anions have been determined. Under comparable conditions, 2-oxocyclohexanecarboxylic acid is halogenated 500–1000 times more rapidly than is its methyl ester. Observed isotope effects (k_H/k_D) in the halogenation of the acid are high and relatively constant for the bases studied, varying between 7·2 for the uncatalysed reaction to 8·0 for acetate catalysis. Corresponding values for the ester vary between 4·2 (uncatalysed) and 7·5 (acetate catalysis). The results indicate a considerable degree of participation of the carboxylic acid group in the halogenation reaction.